Wet method of preparing plutonium tribromide



United States OF PREPARING PLUTONIUM TRIBROMIDE No Drawing. Application December 31, 1947 Serial No. 795,136

6 Claims. (Cl. 23-14 .5)

WET METHOD This invention relates to the preparation of plutonium tribromide, and more particularly to the preparation of plutonium tribromide in the anhydrous form from solution containing either tetravalen't or trivalent plutonium.

An object of this invention is to provide a method for the preparation of the tribromide of plutonium from aqueous solutions containing tetravalent plutonium.

A further object is to provide a method whereby plutonium tribromide in both the hydrated and anhydrous forms can be prepared from aqueous acid solutions containing plutonium.

Other objects and advantages will become apparent on further examination of this specification.

In accordance with the process of this invention plutonium tribromide is prepared from aqueous acid solutions containing plutonium tetrabromide by heating and drying the latter solution in the presence of an excess of bromide anions. The presence of bromide anions in excess simultaneously effects reduction of tetravalent plutonium in aqueous solutions to trivalent plutonium and prevents hydrolysis of the resultant plutonium tribromide to plutonium oxybromide during the drying process consisting of the evaporation of the plutonium-containing solutions and the dehydration of the resultant compounds.

Bromide anions are incorporated in the", aqueous plutonium-contain'ing solutions in excess sufficient to furnish the requisite oxidation-reduction potential, which, in this case, must be substantially more positive than the standard oxidation-reduction potential for the tetravalent-trivalent plutonium couple, i. e., it must be more positive than 0.966 volt. Materials suitable for incorporation in such an aqueous system in order to furnish the bromide anions necessary to provide the more positive oxidationreduction potential are: gaseous hydrogen bromide, a concentrated aqueous solution of hydrogen bromide, or a volatile bromide salt, such as ammonium bromide. An excess of bromide anions may be furnished by dissolving a tetravalent plutonium compound, such as the tetrahydroxide or an oxide of plutonium, in a concentrated (5 M) solution of hydrogen bromide. When a volatile bromine-containing compound, such as ammonium bromide, is used, about a six-fold excess is preferred in order to insure adequate reduction of the tetravalent plutonium in solution. This compound must be removed by volatilization during the drying and isolation of the desired product. Hydrogen bromide is the preferred reducing agent for the process of this invention, since by the use of this compound no impurities are introduced which necessitate removal before obtaining a pure compound.

When the tetravalent plutonium, present in solution as the bromide is heated to 70 C. in the presence of an excess of bromide anions, the plutonium is reduced to the trivalent state simultaneously with the liberation of bromide, which boils out of the solution at this temperature. Such a reaction is represented by the following equation:

In a further embodiment of this invention anhydrous plutonium tribromide is prepared by evaporating a solution containing. either plutonium tribromide or plutonium tetrabromide to dryness in a stream of hydrogen bro mide and heating the resultant hydrated product residue at a substantially elevated temperature in the continued presence of hydrogen bromide.-

It is readily apparent that the salt is obtained in hydrated form by limiting the drying process to the evaporation step alone and the product is obtained in the anhydrous form by further extending the drying process to include dehydration which is carried out at substantially elevated temperatures under the following conditions.

Evaporation of solutions containing a bromide of plutonium in either the tetravalent or the trivalent state is preferably carried out at subatmospheric pressures of the order of 10-15 mm. of hydrogen bromide. The use of moderately elevated temperatures not in excess of C. and preferably a temperature of about 70 C. favorably aflfects both the reduction and the rate of evapo ration. r

In order to obtain the anhydrous product, the hydrated product residue prepared as set forth above is further heated under reduced pressure at a substantially elevated temperature of the order of between 225 and 650 C. in the presence of hydrogen bromide. However, a temperature of about 300 C. is preferred. The anhydrous compound thus produced is crystallographically identical with that prepared by the hydrobromination of the oxide in the dry state, which product is a particularly useful compound in reduction processes designed forthe formation of the pure metal.

The following examples will serve as illustrations of the principles outlined above.

Example I Tetravalent plutonium hydroxide containing 2 mg. of

plutonium is dissolvedin 2 cc. of concentrated (5 M) I hydrogen bromide. Since this amount of hydroxide does not entirely dissolve at room temperature in the 2-cc. volume of hydrogen bromide solution, the mixture was heated to 70 C. after which solution was complete. Reduction of the plutonium from the tetravalent state to the trivalent state is evidenced by the observed color change of this solution to the purple color characteristic for solutions containing plutonium in the trivalent state.

A solution of plutonium tribromide thus obtained was evaporated to dryness in a stream of hydrogen bromide at 15 mm. pressure. The hydrated residue, which has been found by X-ray analysis to be isomorphous with the hexahydrate of neodymium tribromide, was heated gradually in a stream of hydrogen bromide at 15 mm. pressure up to a temperature of about 300 C. for a sixhour period. The anhydrous product was dull green and crystallographically identical with the tribromide of plutonium obtained by the dry preparation.

The melting point of the anhydrous product is 654:4 C. The crystal structure is orthorhombic with four molecules per unit cell. The lattice constants are: a =12.57:0.05 A., a =4.1l -0.03 A., a =O.13i0.04 A. The calculated density is 6.69. Determinations of the space grouping and the atomic positions revealed that each plutonium atom is bounded by eight bromine atoms, the atomic distance being Pu:Br=3.08 A. The structure is the layer lattice type and it is isomorphous with the compounds NdBr and SmBr The anhydrous plutonium tribromide resembles the other trihalides of plutonium in that it is. also hygroscopic and on exposure to moist air forms the hexahydrate which is isomorphous with the hydrated forms of neodymium and plutonium halides. On continued exposure to moist air the compound deliquesces.

is taken to exclude any chlorine impurity from the li'y drogen bromide used in the reaction, about of the halogenv atoms in the crystal .will. be. those .oLchlorine rather than bromine, thus producing la..variation..in. the.-.

crystal lattice constants,,suchas .a =7.806' and-181 4.302";-

which isia hexagonal. crystal rather than "thetrue t rhombic crystal structure. I

. Example LE. I An aqueoussolution of plutonium tribromide, prepared *as "above fromthephydroxide, was; evaporated to dryness in a stream"Ofhydmgenbromi'd'e at.10- mm. pressurein "the presence of a' six fold excess of ammonium bromide! Whenonlyya solid phase" remained,'this system" was evacuatedto'. 10T5 mm. and the temperature was. gradually raised" over a" periodoffour hours to 350"" C5 At. this latter temperature the ammonium bromide sublimed away completely. T he product" analyzed forv 64% plutonium-tribromide, 18%"plutonium .oxybromid'e, and.

limitations 'L PQn' the .scopeflof the invention as defined in ,the .appendedclaims.

'What: isclaimed is:

1. A process for the .preparation .of anhydrous plu-.- tonium' tribromide" from an aqueous acid solution of plutonium tetrabr'omide. which comprises incorporating.

therein a. water-soluble volatile bromide to provide additional'bromide ions in sufficient excess to furnishan oxidation-reduction potential substantially more positive than 0.966 volt, evaporating; the resultant plutonium tribromide todryness in the.presence of. ihydrogenhbromide, anddehydrating the residueat an elevatedtemperature in: the presence .of hydrogen bromide.

The process-.015 claim..1 whereinevaporatiomto dryatmospheric pressure.

'4 ness is'carried'out ata temperature between *andl00 C. and dehydration at from 225 to 600 0., both at sub- L 3. The process of claim 2 wherein the evaporation temperature is about 70 C. and the dehydration temperature about 300 C. and the hydrogen bromide pressure in both steps ranges between 10 and 15' mm. I

4. ,Ai.pr0cess.:1.for .thtepreparation. ofleanhyd'rousJ-plm jg tonium tribromide from anzaqueousz acid solution of plutonium tetrabromide, comprising incorporating in said-:- aqueou's. solution a..volatile,water-soluble bromide capableof. providing.additional fbromide ions in sufiicient excess to furnish a standard oxidation-reduction.potential substantially more positive than -0.966 volt insaid aqueous solution of. plutonium. tetra'bromide', digesting the mixture, evaporating the resultant aqueous solution of plutonium tribromidein thepresence of-hydrogen bromide, and dehydrating the residue by heating at substantially elevated. temperature in the presenceof hydrogen. 'br'omideh p 1 5.. The. process of. claim 4 wherein-the volatile water soluble. bromide-.is-ammonium. bromide. p

6..The process. of claim 4 wherein the volatile Watersoluble bromide is; hydrogen=bromide. 5

ReferencesCited' in the file of this'patent.

UNITED STATES PATENTS H-ulin a May 27,-1'919- 1,304,567. 1,567,317 Cottringer et al. Dec. 29, 1925 FOREIGN PATENTS 7 32,338 Germany '.Ju1 8.188s

OTHER REFERENCES Kleinhekselet al..: J. AC. 8., Vol.50, pp. 959-967, April 1928.

Seahorg et al.:' The-Transuranium Elements, lst ed, Pt. .I,vol. IV l4B,.pp.. 743745,.75.7, 758, 764, 765,772,. L. 773,. 774; McG'raw+Hill. Book v00. Inca, N. Y.', 1949. Noting particularly. page v765 whichdescribes arprocess reterredlto by bibliographic reference 17 as Report CK..- 1586 (May 1, 1944) (A-2251), pp. 7-8. p

Harvey et aL: Journal. of the ChemicaLSociety, 1947 (August), pp. 1010-1021, particularly pp. 1010-11,- p 5 

1. A PROCESS FOR THE PREPARATION OF ANHYDROUS PLUTONIMUM TRIBROMIDE FROM AN AQUEOUS ACID SOLUTION OF PLUTOMIUM TETRABROMIDE WHICH COMPRISED INCORPORATING THEREIN A WATER-SOLUBLE VOLATILE BROMIDE TO PROVIDE ADDITIONAL BROMIDE IONS IN SUFFICIENT EXCESS TO FURNISH AN OXIDATION-REDUCTION POTENTIAL SUBSTANTIALLY MORE POSITIVE THAN -0.966 VOLT, EVAPORATING THE RESULTANT PLUTONIUM TRIBROMIDE TO DRYNESS IN THE PRESENCE OF HYDROGEN BROMIDE, AND DEHYDRATING THE RESIDUE AT AN ELEVATED TEMPERATURE IN THE PRESENCE OF HYDROGEN BROMIDE. 